The present invention relates to a method for producing hollow superconducting cables, in particular, relates to a method for producing hollow superconductive cables applicable to superconducting magnet.
Superconducting cable of hollow type has various advantages of the following respects, that is, (1) cooling medium such as liquid helium and the like can be fed directly to a hollow portion of said cable so as to be capable of cooling the conductor directly through no medium of insulating material; (2) necessary amount of said cooling medium can be reduced to the least; (3) electromagnetic coil having large mechanical strength can be made because of unnecessity for providing the cooling space between layers in case of said cable being coiled; (4) overall space occupied by said coiled cable can be saved to the minimum; and (5) in case of precooling treatment being performed, precooling can be carried out with one and the same hollow cooling system. However, manufacturing of hollow superconducting cable as set forth above has to be accompanied with special processing technique, so that it has hitherto been deemed to be difficult in case of producing long hollow superconducting cables.
It has recently been developed a method for producing hollow type superconducting cables as disclosed in Japanese Patent Application Publication Gazette No. 3631 of 1975 which comprises a step of winding a plurality of fine superconducting wires around the outer periphery of a hollow conductor made of good conductive metal such as copper, etc., and a step of securely fixing said superconducting wires to the hollow conductor with a binding material. Said method is disclosed, as an embodiment, in such a manner that a superconducting wire having the outer diameter of 0.37 mm composed of Nb-Ti-Ta alloy wire of 0.25 mm is diameter covered with copper is wound around a hollow copper body having the inner diameter of 2.5 mm and the outer diameter of 3.5 mm, and then the former is fixed securely to the latter by soldering with an alloy containing Pb-50%Sn. Hence, the foregoing conventional method is defective in production process because of the need of said superconductive wires to be wound around the hollow conductor, said process is applicable to the case of employing superconducting material of alloy such as NbTi, NbZr, or NbTiZr alloy and the like, but is unable to be applied to the case of employing superconducting material belonging to intermetallic compound such as Nb.sub.3 Sn, V.sub.3 Ga, Nb.sub.3 Ga, Nb.sub.3 Ge, Nb.sub.3 Si, Nb.sub.3 Al, Nb.sub.3 (Al.sub.0.8 Ge.sub.0.2), etc. The reason for bringing forth such defects as set forth above in the conventional method lies in that said alloy superconducting material has the property of relatively good workability, while the superconducting material belonging to intermetallic compound is, in general, extremely fragile, and has inferior property in workability, particularly in ducitility and malleability, thus causing difficulties in carrying out the aforementioned process for winding the superconducting wires belonging to intermetallic compound around the hollow conductor.
Now, such a process for producing superconducting material belonging to intermetallic compound is applicable to practical use that a plurality of metallic elements utilized as the material for composing the foregoing intermetallic compound are directly or indirectly brought into contact with one another and are subjected to heat treatment, thereby both of said metallic elements are diffused each other to compose intermetallic compound.
Accordingly, it may be considered such method for producing hollow superconducting cable comprised of intermetallic compound, on the basis of the aforesaid method, that wires composed of at least two kinds of metallic elements for composing intermetallic compound are wound around a hollow conductor in the course of the stage where intermetallic compound is not composed yet, and are fixed securely to said hollow conductor with a metallic binder, subsequently said wires fixed with metallic elements are subjected to heat treatment so as to obtain intermetallic compound. The foregoing method thus proposed, however, is accompanied with drawbacks in respect of such problems as copper pipes and the like to be used as a hollow conductor are apt to be softened under heat treatment, and metallic binder in molten by heat treatment to cause exfoliation of the wires from said hollow conductor, thus the above-mentioned proposed method is difficult in application to practical use.
Now, superconductive material of intermetallic compound, such as Nb.sub.3 Sn, V.sub.3 Ga, Nb.sub.3 Ga and the like has remarkably excellent property in superconducting as compared with the property of NbTi serving as alloy superconductive material as shown in FIGS. 1 and 2, so that it has seriously been required for the development of a method for producing hollow superconducting cables which are applicable to be put to practical use by employing superconducting material belonging to intermetallic compound.
The present invention is achieved in order to comply with the foregoing demand.